First Aspect of Invention and Third Aspect of Invention
Display surfaces of optical articles such as liquid crystal displays and plasma displays are required to cause no significant reflection of light applied from external light sources such as fluorescent lamps from the viewpoint of enhancing visibility. A phenomenon that the reflectance is reduced by covering the surface of a transparent object with a transparent film having a low refractive index has hitherto been known in the art. Therefore, the visibility can be improved by providing an antireflection film utilizing this phenomenon onto the display surface of an image display device. The antireflection film may have a single-layer structure comprising a low refractive index layer having a low refractive index provided on a display surface, or a multilayer structure comprising one or a plurality of medium to high refractive index layers, for further improving the antireflection effect, provided on a display surface and a low refractive index layer provided on the antireflection effect improving layer(s).
The antireflection film having a single-layer structure has a simpler layer construction than the multilayered antireflection film and thus has superior productivity and cost performance. On the other hand, for the multilayered antireflection film, advantageously, the antireflection properties can be improved by combining layer constructions and thus can realize high performance than the antireflection film having a single layer structure. Further, damage to the surface of an optical article deteriorates the visibility, and, thus, it is common practice to impart hard properties to the antireflection film. Furthermore, an optical article formed of plastic has insulating properties and thus is electrified upon exposure to static electricity and the like, and the adherence of dust onto the surface deteriorates the visual field. This has led to a demand for imparting antistatic properties to optical articles.
An antistatic antireflection film produced by forming a metal oxide-containing antistatic layer on a transparent base material film, further forming a hardcoat layer on the antistatic layer, and forming, as the uppermost layer, a low refractive index layer having a lower refractive index than the underlying layer is known as a film to which these antistatic properties and hard properties have been imparted to antireflection films (for example, Japanese Patent Laid-Open No. 255403/2001).
Japanese Patent Laid-Open No. 255403/2001 describes an antireflection film comprising an antistatic layer which uses a metal oxide or a metal as an antistatic material from the viewpoint of preventing a deterioration in visual field caused by the adherence of dust on the surface of displays. Since metal oxides or metals generally have a higher refractive index than a binder resin, an antistatic layer with a metal oxide or a metal added thereto has a higher refractive index than the base material film or the hardcoat layer and, thus, a refractive index difference occurs between the base material film and the antistatic layer or between the hardcoat layer and the antistatic layer. The refractive index difference poses a problem that interference fringes, which deteriorate the visibility of optical articles such as displays, occur.
For example, for an example of an antireflection film comprising a conventional general antistatic layer, a triacetylcellulose film (a transparent base material film) has a refractive index of about 1.5, a metal oxide-containing or metal-containing antistatic layer has a refractive index of about 1.57 to 1.70, and the hardcoat layer has a refractive index of about 1.50. This great difference in refractive index between layers in contact with each other causes a problem that external light which entered from surface side is reflected from the interface of the transparent base material film and the antistatic layer and the interface of the antistatic layer and the hardcoat layer and the reflected lights cause interferences which are sometimes observed as uneven interference (color shading).
Accordingly, there is an urgent need for the development of a composition for the formation of an antistatic layer which can effectively prevent the occurrence of interference fringes and dust adherence, and an antistatic layer for use in an antireflection film formed using this composition.
Second Aspect of Invention and Fourth Aspect of Invention
Display surfaces of optical articles such as liquid crystal displays and plasma displays are required to cause no significant reflection of light applied from external light sources such as fluorescent lamps from the viewpoint of enhancing visibility. For antireflection purposes, it is common practice to apply an antireflection film, prepared by forming a low refractive index layer having a lower refractive index than the underlying layer either, onto a transparent base material film directly or through other layer, onto the surface of an optical article. Further, damage to the surface of an optical article deteriorates the visibility, and, thus, it is common practice to impart hard properties to the antireflection film. Furthermore, an optical article formed of plastic has insulating properties and thus is electrified upon exposure to static electricity and the like, and the adherence of dust onto the surface deteriorates the visual field. This has led to a demand for imparting antistatic properties to optical articles.
An antistatic antireflection film produced by forming a hardcoat layer on a transparent base material film, forming a metal oxide-containing antistatic layer on the hardcoat layer, and forming, as the uppermost layer, a low refractive index layer having a lower refractive index than the underlying layer is known as a film to which these antistatic properties and hard properties have been imparted to antireflection films (for example, Japanese Patent Laid-Open No. 267804/2002). In the antireflection film disclosed in Japanese Patent Laid-Open No. 267804/2002, the refractive index of the antistatic layer is described to be not less than 1.60 and not more than 1.75.
The refractive index of the antistatic layer containing a metal oxide as an antistatic agent is generally in the range of about 1.57 to 1.75. When an antireflection film having a laminate structure of transparent base material film/hardcoat layer/antistatic layer/low refractive index layer (refractive index n=not more than 1.42) is formed by designing a low refractive index layer so that the minimum reflectance is provided at a wavelength of 550 nm, and stacking the conventional low refractive index layer (n=not more than 1.42) onto an antistatic layer having a refractive index of about 1.57 to 1.75, due to a large difference in refractive index between both the layers, a V-shaped reflectance curve is provided. That is, the reflectance in a short wave area and the reflectance in a long wave area in a visible light region are high, leading to a problem that a reddish/bluish color occurs in the antireflection film and the strength of color at the time of reflection increases with increasing the refractive index difference. Such coloring is particularly significant when the antistatic layer is a thin film having a thickness of 50 nm to 150 nm and, at the same time, the low refractive index layer is a thin film having a thickness of 50 nm to 150 nm.
FIG. 1 is a reflectance curve in which the reflectance (%) is plotted as ordinate and the light wavelength (nm) is plotted as abscissa in the case where, in an antireflection film having a laminate structure of transparent base material film/hardcoat layer/antistatic layer/low refractive index layer, the low refractive index layer has a refractive index (n1) of 1.37 and the antistatic layer has a refractive index (n2) on levels of 1.53, 1.57, 1.61, and 1.65. In FIG. 1, a curve indicated by a solid line shows a reflectance curve of the antireflection film comprising no antistatic layer. As is apparent from FIG. 1, the larger the difference in refractive index between the antistatic layer and the low refractive index layer, the larger the curve in the V-shaped curve and the higher the reflectance in the short wave area and the long wave area and, thus, the stronger the reddish/bluish color in the antireflection film.
Accordingly, there is an urgent need for the development of an antireflection film comprising an antistatic layer having a laminate structure of transparent base material film/hardcoat layer/antistatic layer (thin film)/low refractive index layer (thin film) in which coloring at the time of reflection has been prevented.